Differential amplifiers, and especially error amplifiers, are widely used in control and regulation systems where it is desired to control the rise of the output voltage at the time power is applied. This may be important in DC-to-DC converters, for example, to avoid an overshoot of the regulated output voltage during the start-up phase. In control and regulations systems, differential or error amplifiers typically include an output stage which, in turn, includes a pair of transistors operating in a push-pull mode.
For example, the pull-up transistor is typically connected between the output node and the supply, and the pull-down transistor is connected between the output node and ground. The output node is often also typically connected to the input of a comparator. At start-up, an out-of-balance condition of the inputs of the amplifier may cause a sudden saturation rise of the output node toward the supply voltage. A steep rise of the output voltage toward the supply may cause undesired over-shoot of the value controlled by the regulating system, such as the output of the DC-to-DC converter.
As disclosed in U.S. Pat. No. 5,532,645 to Fagnani et al., the voltage rise at the output node of the error amplifier at start-up has been slowed by introducing a circuit for controlling the voltage rise on an internal driving node of the amplifier or on the output node. For example, the output has been tied to a voltage of a so-called "soft-start" capacitor which is charged with a constant current. The patent further discloses that one previous approach is based on using a PNP transistor with an emitter connected to the output node, a collector connected to a common ground node of the amplifier, and a base connected to an external soft-start capacitor. The external soft-start capacitor may be charged through a charge path that is functionally external to the structure of the amplifier. The PNP transistor prevents too steep a rise of the voltage on the output of the amplifier.
Unfortunately, this approach had several drawbacks. It is generally desired to reduce the size of the external soft-start capacitor, to reduce the current consumption. A diode or transistor that is capable of producing a base current of a few milliamperes, may often be incompatible with a soft-start capacitor requiring a charge current of a few milliamperes. In other words, the base current of the transistor may represent an excessive contribution to the charge current of the soft-start capacitor.
Accordingly, U.S. Pat. No. 5,532,645 discloses a significant advance over the prior control circuits. The patent discloses a circuit for tying the output voltage of an amplifier using a small size soft-start capacitor, while keeping the total charge current to only a few microamperes. The circuit includes a feedback path to control the driving level of the pull-up transistor in the amplifier's output stage. The transistor outputs the charge current for the soft-start capacitor to thereby limit the charge current delivered through a diode to the external capacitor connected to a dedicated soft-start pin of the integrated device. The feedback circuit includes a pair of complementary current mirrors, through which the charge current that flows through the charge diode to the external soft-start capacitor is mirrored in a suitable ratio. The current is mirrored on the base of the same transistor of the push-pull output stage of the amplifier that delivers the charge current to thereby partially reduce its drive condition.
Within the control loop, a certain current balance is achieved such that the current forced by the respective bias current generator of the output transistor that delivers the charge current of the output node is counterbalanced, at least partially, by the current that is a function of the active charge current which is fed to the base of the transistor through the current mirror of the feedback circuit. Accordingly the output node gradually charges itself toward the supply voltage.
An auxiliary current generator may be employed for ensuring a complete charge of the external soft-start capacitor which, at start-up, the output node of the amplifier stabilizes at a certain level imposed by the regulating chain including the amplifier. In any case, upon reaching a fully charged state by the external soft-start capacitor, the charge diode which is part of the first current mirror cuts itself off, thus isolating the control soft-start circuit from the functional circuit of the amplifier.
Unfortunately, the circuitry required for the soft-start function as disclosed in U.S. Pat. No. 5,532,645 and based upon driving from the output of the amplifier may be relatively complicated. In the past and as disclosed in the patent, it was also thought that attempting to provide the soft-start feature on an internal driving node of the amplifier would present severe difficulties, since only a few millivolts may be sufficient to completely unbalance an amplifier.